LRP1 is a member of the low density lipoprotein (LDL) receptor family, which plays important and wide-ranging roles in physiological processes as diverse as lipoprotein metabolism, uptake of proteinases and their inhibitor complexes, viral entry, and even early development. These roles can also lead to disease in instances of mutation and absence, with defects in LDLR resulting in hypercholesterolemia, and the LRP1 knockout being embryonically-lethal in mice. LRP1, like all members of the LDLR family, is a mosaic protein composed of multiple copies of ligand-binding (CR domain), EGF-like and 2-propellor (YWTD) domains. For it to function effectively in vivo it must not only fold correctly and be efficiently exported to the cell surface, but must be able to both bind and internalize extracellular ligands and subsequently release them intracellularly for further processing or degradation. Our long term goal is to understand the relationship between the structure of LRP1 and how it functions. This renewal application seeks quantitative answers to these basic questions, and builds on many years' experience examining the ligand-binding CR domains of LRP1 and the structure and properties of the chaperone protein RAP (receptor associated protein). The current proposal is a logical extension of our reductionist approach of examining pair-wise interactions between binding partners, in which we will utilize insight gained previously on studies of small fragments of LRP1 to test ideas on how complete functional regions of the intact receptor work, but includes both in vitro and parallel in vivo studies. We will address the following specific questions. First, does one RAP molecule exploit its three-domain structure to bind extremely tightly in the ER to each ligand-binding cluster of LRP1 and so protect it from premature binding of any other potential ligand? Second, is this same multi- domain high affinity interaction of RAP exploited to promote correct folding of the ligand-binding clusters in the ER? Third, do the YWTD domains that flank each of the ligand binding clusters serve to displace both internalized ligands and RAP by a common competition mechanism at low pH that is made highly efficient by the intramolecular nature of the competing domain?